In Firefox 51 (using macOS 10.12.3), LastPass from the official Mozilla add-on site (LastPass v. 3.3.2) disables Firefox e10s multiprocess entirely (shown by both about:support and the Add-on Compatibility Reporter). Downloading LastPass directly from lastpass.com, currently v. 4.1.35 is shown as being e10s compatible, and works for that session, however, when Firefox is restarted, Firefox permanently disables that version of LastPass as not being verified. Mozilla support indicates that the only fix to an unverified add-on is to contact the developer. This was the case under FF 50 as well. So the current option is use FF and LP 3 with e10s disabled, which is very slow compared to Chrome/Safari, or reinstall LastPass 4 everytime FireFox is restarted. If this isn't fixable I'll just switch browsers, LastPass works fine everywhere else.